How does a varicocele cause infertility?

We are certain that varicoceles decrease fertility but we have not yet
categorically determined why this is so. There are several theories:

A) Increased temperature of the testicles

The testicles are located in the scrotum, which effectively regulates
their temperature. They are maintained at a temperature slightly below body
temperature. (This is probably why they are located outside the body rather
than inside the body where they clearly would be better protected.) In cold
weather you may notice that a man's testicles move close into his body as
the cremasteric muscles, the muscles in the scrotum wall, tighten. In warm
weather the cremasteric muscles relax and lengthen allowing the testicles to
hang away from a man's body and cool down.

Some babies are born without their testicles having descended into their
scrotum. They are trapped somewhere in their bodies and constantly exposed
to body temperature. This is so harmful for the testicles that if they
remain there past puberty they will stop producing sperm altogether and have
a higher chance of developing cancer. Therefore if a boy's testicles do not
descend into the scrotum by the time he is 12 months old, they should be
surgically brought down and placed into the scrotum.

Varicoceles are a group of dilated veins filled with blood, which
surround the testicles. The blood is at body temperature and if the testes
are near these veins they will be kept at a higher temperature than is
beneficial for them. Even if a man has a varicocele only on one side, the
whole scrotum is warmed by the blood and both testicles can be negatively
affected.

In general, larger testicles make more sperm than smaller testicles.
Often however, you see men who have a large one-sided varicocele that has
damaged the testis on one side making it smaller. The small teste makes
significantly less sperm than the normal one. However even in the "normal"
one the sperm quality is often very low. The varicocele is not only damaging
the teste on the side where it is found but also suppressing the sperm
production on the opposite (better) side.

When a varicocele is repaired the blood is no longer able to flow back
into the scrotum. This affects not only the testes on that side, but also
the opposite side with this normalization of temperature, there may be some
dramatic improvement in sperm production. This improvement is most likely
mostly coming from improved production in the larger better testicle.

B) Increased waste products back-flowing into the testicle

The veins draining the testicles connect into larger veins. On the left
side, they drain into the kidney vein, which is draining blood from the
kidney. The blood from the kidney carries waste products, which may then
drain backwards into the scrotum and collect there. This may negatively
effect sperm production.